The present invention relates to sensing temperature, for example for a tool for forming a composite component.
It is known to make composite components of composite materials using a tool having one or more heaters embedded in the tool. It is necessary to monitor and/or control the temperature of the tool during a cure cycle. Normally thermocouples have been used but they have several disadvantages. A single thermocouple monitors only a localised area and many thermocouples are needed on a larger tool resulting in multiple wires and connections. Normally, thermocouples are placed just outside the edge of the component being formed in “spare” material which is removed from the component after it is formed. Furthermore, thermocouples and their wiring may embed themselves into the material being formed resulting in damage to, and replacement of, wiring after the composite material has been removed from the tool, and recalibration of the thermocouples if they are reused. Thermocouples are not used to directly monitor the material of the component itself (i.e., the material which is not discarded) because they may become embedded in the component or otherwise mark the surface of the component.
A fibre optic sensor using a Bragg grating has been proposed for sensing a variable for example temperature or the concentration of a chemical species. It is known to provide many Bragg gratings distributed on an optical fibre to sense strain on the fibre. It is also known to use Wavelength Division Multiplex or Time Division Multiplex techniques to address the respective gratings on the fibre.
WO 00/39548 discloses an array of gratings distributed along a fibre, each grating being on a portion of fibre encased within, and fused to, a cylindrical glass tube, which may have a coefficient of thermal expansion mis-matched with that of the fibre to increase the sensitivity of the grating to temperature changes by applying strain to the grating. The glass tube also reduces the sensitivity of the grating to axial strain on the fibre outside the tube. However, because the tube is fused to the fibre, axial strain on the fibre is transmitted to the fibre within the tube so the value sensed by the grating within the tube is not independent of axial strain on the fibre.
WO 03/076887 discloses the use of an optical fibre that includes one or more Bragg gratings that are responsive to strain and are for use in large structures such as ships, bridges and oil drilling and production rigs. This document also discloses that a Bragg grating can be placed in a package that isolates the grating from strain with the purpose of providing a temperature measurement that can be used to compensate for the thermal signal measured by the strain responsive grating. Accordingly, WO 03/076887 describes the use of a Bragg grating to provide for temperature compensation, rather than for actual temperature measurement. One example of a strain isolating package disclosed in WO 03/076887 receives a loop of the optical fibre in a circular groove in a disc, the optical fibre entering and exiting the circular groove tangentially to the circular groove. This strain isolating package has the disadvantages that it is relatively large and further involves the optical fibre intersecting, or crossing over, itself where the optical fibre enters and leaves the circular groove. Another example of a strain isolating package, described as being to provide a more compact arrangement, involves a straight length of the optical fibre with the Bragg grating being placed on a high elastic modulus material, a stiffener and fixed to the stiffener using a high modulus adhesive. The optical fibre and stiffener a further encapsulated in a lower elastic modulus polymer to reduce strain concentrations in the optical fibre at either end of the stiffener. This strain isolating package has complex construction.
Accordingly, the invention seeks to provide a temperature sensor that has a compact and inexpensive construction and that is useful in a tool for forming a component of composite materials and to provide a temperature sensor array and a tool incorporating such a temperature sensor or a temperature sensor array.